Kerckhoffs' Principle (19th c.)

This has a number of implications:

The system is at most as secure as the number of keys
Eve can just try them all, until she finds the right one

A strong cryptosystem is one where that's the best Eve can do
With weaker systems, there are shortcuts to finding the key

Example: newspaper cryptogram has 403,291,461,126,605,635,584,000,000 possible keys
But you don't try them all; it's way easier than that!